feat(web): hide subagent threads from user-facing thread lists (5/5)

[shivamhwp]

Last of five stacked PRs replacing #4551. Stacked on #4663 — review that first.

A subagent's child thread is an implementation detail of the agent that owns it, not a conversation the user started. Listing them alongside real threads makes the sidebar grow with every delegation.

Filtered at the source — the shell snapshot the server serves — so every client sees the same set rather than each re-deriving it. The web client filters again for locally-held state, and the thread route redirects to the parent if a subagent thread is reached directly.

Mobile is part of the sweep

Found in review, and the reason this isn't a web-only change. Mobile still offered "Open subagent thread" with no redirect to fall back on, so it would have navigated to a thread the client no longer receives. Its relationships banner also needed the same subagent-edge filter web has — those edges derive from the projection rather than from thread shells, so they survive the shell filtering and would have rendered as "Unavailable", and by sorting first could have taken over the banner headline.

The trade-off, stated plainly

A subagent's transcript becomes unreachable. The Agents panel from #4663 shows what an agent did and what it returned, but not its full conversation. That is the intended shape here, and the "Open subagent thread" affordances are removed on both clients rather than left as dead ends — but it is a real capability loss and worth a deliberate decision rather than being smuggled in with a sidebar tidy-up.

Testing

Typecheck clean on server, web, client-runtime. Server 1,685 passing, web 1,500 passing, client-runtime 485 passing. (apps/mobile has 70 pre-existing typecheck errors on the base branch, unchanged by this PR.)

Known, not fixed here

• The route's redirect-to-parent is unreachable against a server that also filters, because the shell is already absent and the earlier "missing thread" effect wins — a deep link to a subagent thread lands on / rather than the parent. It matters under version skew, so it is kept.
• entities.ts swaps memoized ref atoms for per-call filter+map. The cost is negligible but the identity stability is not; filtering inside the client-runtime atoms instead would be the better shape.
• Each shell event on a subagent thread now emits a spurious thread.removed delta, absorbed harmlessly by the reducer.

Note

Hide internal subagent threads from all user-facing thread lists and navigation

• Adds a new threadVisibility.ts module with isInternalSubagentThread and isUserFacingThread helpers that classify both lineage-subagent and node-owned subagent threads as internal.
• Filters subagent threads from HTTP and WebSocket shell snapshot responses via a new userFacingShellSnapshot utility in ThreadManagementService.ts.
• Removes subagent relationship links and open-thread buttons from the sidebar, relationships panel, message timeline, and work log across web and mobile.
• Redirects direct navigation to an internal subagent thread to its parent thread (or /) in _chat.$environmentId.$threadId.tsx.
• Risk: findThreadRef now returns nothing for internal subagent thread IDs, which could silently break any code that attempts to look up a subagent thread by ID.

📊 Macroscope summarized 6bc3a8d. 12 files reviewed, 0 issues evaluated, 0 issues filtered, 0 comments posted

(Automatic summaries will resume when PR exits draft mode or review begins).

🗂️ Filtered Issues

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